1. Field of the Invention
The present invention generally relates to transmission methods for communication systems supporting a multicast mode.
2. Description of Related Art
Traditionally, voice communication has been the dominant application in a wireless networks. As a result, cellular standards, such as Global System for Mobile Communication (GSM) and IS-95 were optimized for voice traffic only. With the recent explosive growth of the Internet, however, a need has arisen to offer reliable voice and high-speed data access over wireless networks. Until recently, standardized 3rd Generation (3G) systems such as CDMA2000 and Enhanced General Packet Radio Service (EGPRS) attempted to provide such capability, by evolving the air interface of existing voice-centric, second generation (2G) systems. The service needs of voice and packet data, however, are different.
The support of delay-tolerant data services in these standards has proven to be inadequate, centrally because voice-centric techniques were applied to resource allocation for packet data. The recently standardized CDMA2000 1X EV-DO supports efficient packet data service over a dedicated CDMA2000 1X carrier by using a design philosophy different from that of CDMA2000 and EGPRS, resulting in a substantially superior performance. However, 1X-EV-DO is not backward compatible with existing 1X systems and does not support voice service on the same carrier.
Thus, an expanded effort is underway in the 3rd Generation Partnership Project (3GPP) and 3GPP2 for the evolution of 3G systems such as the Universal Mobile Telecommunications System (UMTS) and CDMA2000 1X. These 3G evolutions, reflected in the high-speed downlink packet access (HSDPA) system in UMTS and in the recent 1X EV-DV standards, have begun to address the challenges of supporting the separate and often conflicting needs of voice and high-speed data simultaneously and efficiently on the same carrier, in a manner that may be fully backward compatible.
To meet the rapidly developing needs associated with wireless applications such as wireless internet application, and to support HSDPA, a shared channel called a High-Speed Downlink Shared Channel (HS-DSCH) may be used. The HS-DSCH may be enabled by a number of performance enhancing technologies, such as Fast Scheduling, Adaptive Modulation and Coding (AMC) and Hybrid Automated Repeat Request (HARQ). Fast Scheduling is a channel quality sensitive scheduling technique to maximize sector throughput, e.g., a base station assigns resources to one or more users at a given time based on channel quality. AMC technologies enable a selection of a data rate and a transmission format (i.e., modulation level and channel coding rate) that best “suits” the scheduled user's prevailing channel condition.
Delays and measurement errors may result in degraded performance from AMC. For example, suppose a block of bits or a packet was sent out using QPSK modulation and a code rate of 0.5 and was received erroneously. A retransmission of that packet takes place, in general with a new appropriate choice of modulation and in general, at least a few new “parity” bits from the original set of coded bits. HARQ technologies may thus be used to provide some level of robustness through fast retransmissions at the physical layer, in an attempt to minimize degradation.
HARQ allows combining of the original transmission with the new transmission, rather than to discard the original transmission. This greatly improves the probability of correct decoding of the packet. The word “hybrid” in HARQ indicates that Forward Error Correction (FEC) techniques have been used in addition to ARQ techniques. HARQ combining schemes imply that retransmissions are combined with the original unsuccessful transmissions. Accordingly, HARQ helps to ensure that transmissions resulting in unsuccessful decoding, by themselves, are not wasted.
There are two types of HARQ: type-I and type-II HARQ. In type-I HARQ, the sender retransmits the same packet upon the reception of a negative acknowledgement (NACK). In type-II HARQ, the information message is encoded into a number of distinctive packets. Upon the reception of NACK, an incremental packet is transmitted by the transmitter. The receiver then combines the packet with the previous packet to jointly decode the message. Lucent Technologies Inc.'s Adaptive Asynchronous Incremental Redundancy (A2IR) approach, which is a form of HARQ combining (type-II HARQ), has been adopted in both 1X EV-DV and HSDPA. A2IR is a flexible approach and allows HARQ combining of copies that use potentially different modulation schemes.
In UMTS, it is envisaged that for some applications, multiple users should be able to receive the same data at the same time. Two services have so far been defined: (1) a cell broadcast service (CBS); and (2) an IP-Multicast service. A CBS allows for low bit-rate data to be transmitted to all subscribers in a set of given cells over a shared broadcast channel. This service offers a message-based service. An IP-Multicast service enables mobile subscribers to receive multicast traffic. This service currently does not allow for multiple subscribers to share radio or core network resources, and as such, does not offer any advantages as far as resource utilization within the Public Land Mobile Network (PLMN) and over the radio access network.
Multicast is a bandwidth-conserving technology in UMTS that reduces traffic by simultaneously delivering a single stream of information to a large number of recipients. When there are N users in a multicast group, the resources consumed in accordance with a multicast scheme should be on the order of 1/N that of a unicast scheme. Upon the reception of a data packet, a receiver generates a acknowledgement (ACK)/negative acknowledgment (NACK) packet that is sent to the transmitter, indicating whether the transmission is successful or not. In multicast services, these ACK/NACK packets may cause severe collisions or severe interference, if the number of receivers in a multicast group is substantial. Thus, in communication systems employing a multicast mode, when error requirements of a message are stringent, ARQ protocols such as type-I and type-II HARQ (A2IR) may need modification in order to reduce transmission errors over communication links.